I live just along the lake from you, in Lausanne, and also have a tandem (two now in fact, one is for sale - see below). There arre no good local sources that I know of for tandem-specific parts, SJS Cycles in the UK is the most convenient online retailer that carries tandem parts. You might be able to get some derailleur cables at your local bike shop that are packaged as standard "Dura Ace" cables, which are in fact tandem length - we sell some of those in the shop I'm working in, plus we have some dedicated tandem length brake cables for sale - the store is called The Bike in Pully, just on the edge of Lausanne.

The shop did have a couple of Cannondale tandems that we used to rent, but they needed some serious work. I bought one for cheap "as is" and have now finished overhauling and refurbishing it, so if you want a completely upgraded bike, then I have a Cannodale Road tandem in size Large/Small (see chart here) for sale for 1500 Swiss Francs, it is about 8 or 10 years old and has had intermittent use during that time due to being a rental.